It is well-known that in many semiconductor devices, such as VD-MOST and SIT, a high sustaining voltage always accompanies a high specific on-resistance. This is due to the fact that, for a high sustaining voltage, thickness of a voltage sustaining layer should be large and doping concentration of the voltage sustaining layer should be low, so as the peak field does not exceed the critical field for breakdown—EC, which is normally expressed by EC=8.2×105×VB−0.2 V/cm for silicon, where VB is the breakdown voltage of the voltage sustaining layer.
In a uniformly doped n-type voltage sustaining layer between p30 -region and n+-region, in order to obtain a minimum specific on-resistance at a given breakdown voltage; a doping concentration ND and a thickness W of the voltage sustaining layer are optimized such that a maximum field is at p+-n-junction and its value is equal to EC, a minimum field is at n+-n-junction and equal to EC/3. For silicon device,ND=1.9×1018×VB−1.4cm−3   (1)W=1.8×10−2×VB−1.2μm−2   (2)(see, e.g., P. Rossel, Microelectron. Reliab., vol. 24, No. 2, pp 339-336, 1984).
In a VDMOST shown in FIG. 1A, a field profile in the voltage sustaining layer at VB is shown in FIG. 1B, where a slope of the field versus distance is qND/ES,ES is the permittivity of the semiconductor and q is the electron charge. The change of field through the n-region is qND/ES, 2EC/3. The relation between Ron and VB of a n-type voltage sustaining layer is then expressed byRon=W/qμnND=0.83×10−8×VB2.5Ω.cm2   (3)where μn is the mobility of the electron and μn=710×VB0.1 cm/V .sec is used for silicon.
In order to get even lower Ron at a given VB, some research have been done to optimize the doping profile instead of using a uniform doping, see: [1] C. Hu, IEEE Trans. Electron Devices, vol. ED-2, No. 3, p243 (1979); [2] V. A. K. Temple et al., IEEE Trans. Electron Devices, vol. ED-27, No. 2, p243 91980); [3] X. B. Chen, C. Hu, IEEE Trans. Electron Devices, vol. ED-27, No. 6, p985-987 (1982). However, the results show no significant improvement.